1. Field of the Invention
The present invention relates to a piezoelectric transformer used for the inverter for lighting the backlight of a liquid crystal display, the inverter for lighting a fluorescent lamp and the high-voltage power circuit of a copier or the like, and the piezoelectric inverter and the liquid crystal display using the same.
2. Description of the Related Art
As a well known piezoelectric transformer, there is a Rosen type piezoelectric transformer comprising a driving section wherein a pair of input electrodes are formed on a rectangular planar piezoelectric plate and which is polarized in the thickness direction, and a power generation section wherein an output electrode is formed on the end face in the longitudinal direction and which is polarized in the longitudinal direction. This type of piezoelectric transformer is constituted so as to mechanically vibrate the piezoelectric plate by applying an input voltage to the input electrodes via the input side wiring, and to take out the output voltage of AC high voltage generated at the output electrode via the output side wiring. As a lead for each the above-mentioned wiring, a lead that least inhibits vibration and that is not broken by vibration is desired. There is proposed, therefore, a piezoelectric transformer using, as leads for wiring, gold thread wires which have a flexibility and a good vibration-following property (Japanese Unexamined Patent Publications No. 8-32139 and No. 8-64884).
Conventional gold thread wires used as leads for wiring of a piezoelectric transformer are classified into three types as follows: a type having a structure wherein a copper foil (metallic foil) 7b in the shape of band is singly wound spirally around a core 7a consisting of numerous fine wires bundled, as shown in FIG. 6A and 6B; a type having a structure wherein two copper foils 7b are doubly wound around the core 7a in the same direction to increase the mechanical strength (bending strength), as shown in FIG. 7A; and a type having a structure wherein two copper foils 7b are doubly wound around the core 7a in the directions such that the two copper foils intersect each other, as shown in FIG. 7B. These gold thread wires are soldered at the input electrodes or the output electrode for connection.
However, in the conventional piezoelectric transformer using the above-described gold thread wires, there has been problems that, when a gold thread wire contacts or approaches the piezoelectric plate or a mounting board in the vicinity of the gold thread wire, it vibrates concurrently with the vibration of the piezoelectric transformer during operation, thereby causing soundings, and wearing copper foils of the gold thread wire, with the result that it becomes thin, and incurs a bad electrical conduction, or even a breaking under certain circumstances.
Further, there has been another problem that, when using a gold thread wire with doubly-wound copper foils, the friction between copper foils causes the copper foils to wear, thereby incurring the thinning or breaking of the wire.
Moreover, because the copper foil of a conventional gold thread wire directly contacts air, the copper foil is oxidized with time, and thereby its solderability (wettability toward solder) deteriorates, so that adequate fillets for soldering may be not formed, providing a bad soldering.
Also, there have been a problem that, when soldering a conventional gold thread wire, solder is easily wicked to the outer peripheral surface and the inside of the gold thread wire, thereby the solder wicked impairing the flexibility of the gold thread wire, and inhibiting (suppressing) the vibration of the piezoelectric transformer, which results in the deterioration of the performance of the piezoelectric transformer.
In recent years, there is an growing need for the size reduction of equipment using a piezoelectric transformer, and accordingly, the size reduction of the piezoelectric transformer itself is also strongly demanded. Now that the shortening of the gold thread wire for wiring having thus become a necessity for the piezoelectric transformer, the above-described problems are increasingly manifesting themselves.